Carbon Nanotubes for Solar Energy (2/2)
Carbon nanotubes in photovoltaics
Don’t you love it when the sun wakes you up early in the morning? The sun is kinda like this:
Well, so is Solar Energy except we’re using the sun’s energy to harness power, electricity or energy.
In my previous article, I talked about what these reallyyyy (nano) tiny tubes of Carbon Sheets; Carbon Nanotubes (SWCNTs) are, how they’re made and some of there applications. Check that out here.
Carbon Nanotubes have some really unique properties which allow it to be used for various different applications. One of the most promising applications of Carbon Nanotubes would be for next-generation solar cell applications.
Today, let’s talk more about how Carbon Nanotubes can be used in photovoltaic devices, for light harvesting, and dye-sensitized solar cells.
Carbon nanotubes in photovoltaics
Organic photovoltaic devices (OPVs) are created from thin films of organic semiconductors like polymers and are typically just 100 nm thick.
What makes OPV technology unique is its ability to be utilized in a large area and flexible solar modules. Manufacturing cost is also reduced for organic solar cells because of their lower cost compared to silicon-based materials.
Because polymer based OPVs can be made using a coating process such as spin coating or inkjet printing, they are a very common option for inexpensively covering large areas as well as flexible plastic surfaces. It is a low-cost alternative to conventional solar cells made of crystalline silicon.
Plug time: If you want a brief overview of how Photovoltaics work, check out this article.
Now, here’s a quick rundown of how Organic photovoltaics works:
Organic Photovoltaic (OPV) devices convert solar energy to electrical energy. A typical OPV device consists of one or several photoactive materials sandwiched between two electrodes.
In an OPV cell, sunlight is absorbed in the photoactive layers composed of donor and acceptor semiconducting organic materials to generate photocurrents. The donor material (D) donates electrons and mainly transports holes and the acceptor material (A) withdraws electrons and mainly transports electrons.
- Those photoactive materials harvest photons from sunlight to form excitons, in which electrons are excited from the valence band into the conduction band (Light Absorption).
- Due to the concentration gradient, the excitons diffuse to the donor/acceptor interface (Exciton Diffusion) and separate into free holes (positive charge carriers) and electrons (negative charge carriers) (Charge Separation).
- A photovoltaic is generated when the holes and electrons move to the corresponding electrodes by following either donor or acceptor phase (Charge Extraction).
Carbon nanotube composites in the photoactive layer
But first, let’s learn about PN Junction.
The PN junction is a junction formed together by P-type semiconductor and N-type semiconductor material. The junction plays an important role in the development of the diode which is the building block of most of the semiconductor devices like transistors and solar cells.
- The PN junction is generated when one side of the junction is doped with acceptor impurity (trivalent) in the positive region (P-type) and another side is doped with donor (pentavalent) impurity in the negative region(N-type).
- The region where both N-type and P-type material are joined together is called junction (or boundary of the semiconductor).
- The majority charge carriers in the P region are holes, and major charge carriers in N-region are electrons.
- Key Idea: When the junction is formed, the holes present in the P region are diffused into the N-region, leaving behind the negative charge in the P region which then recombines with electrons, resulting negatively charged ions in the P-region.
- Key Idea: In this same way, electrons present at the N-region diffuse into the P-region leaving behind the positive charge in the N-region and recombine with the holes, creating positively charged ions in the N-region.
- As the junction is formed each region of silicon crystal becomes depleted from major charge carriers around the junction. This region is known as depletion or space charge region. The width of this region highly affects the current flowing between the junction.
Now, what is Conjugation?
In organic chem, Conjugation is used to describe the situation that occurs when π systems (e.g. double bonds) are “linked together”.
- An “isolated” π (pi) system exists only between a single pair of adjacent atoms (e.g. C=C)
- An “extended” π (pi) system exists over a longer series of atoms (e.g. C=C-C=C or C=C-C=O etc.).
- An extended π (pi) system results in an extension of the chemical reactivity.
The fundamental requirement for the existence of a conjugated system is revealed if one considers the p orbitals involved in the bonding within the π system.
- A conjugated system requires that there is a continuous array of “p” orbitals that can align to produce a π bonding overlap along the whole system.
- If a position in the chain does not provide a “p” orbital, then the conjugation is interrupted (broken).
The result of conjugation is that there are extra π bonding interactions between the adjacent π systems. This extra bonding results in an overall stabilization of the system. This increased stability due to conjugation is usually referred to as the resonance energy or conjugation energy.
Conjugated polymers have unique material properties because of their ability to virtually create new materials by simply tuning the molecular structure.
We can combine the physical and chemical characteristics of conjugated polymers with the high conductivity along the tube axis of carbon nanotubes (CNTs) to disperse CNTs into the photoactive layer, to obtain more efficient OPV devices. Photovoltaic efficiency enhancement is gonna be needed due to the internal polymer/nanotube junctions within the polymer matrix. The high electric field at these junctions can split up the excitons, while the SWCNT can act as a pathway for the electrons.
The dispersion of CNTs in a solution of an electron donating conjugated polymer is the best way to introduce CNT materials into OPVs.
Single Wall carbon nanotubes (SWCNTs) for light
The photovoltaic effect can be achieved in ideal single wall carbon nanotube (SWNT) diodes. The PN-junction diode acts like solid state one-way electrical valve that only allows electrical current to flow through themselves in one direction only. The advantage of this is that diodes can be used to block the flow of electric current from other parts of an electrical solar circuit.
Individual SWNTs can form ideal p-n junction diodes. Under light, SWNT diodes have strong power conversion efficiencies like the enhanced properties of an ideal diode.
A diode is normally made by joining a p-type semiconducting material — which has been doped with impurities to add extra “holes” — to an n-type semiconductor that has an excess of electrons. However, it is also becoming possible for us to do this in a carbon nanotube if we use an electric field to create the p and n regions instead.
Recently, SWNTs were directly configured as energy conversion materials to fabricate thin-film solar cells, with nanotubes serving as both photogeneration sites and a charge carriers collecting/transport layer.
The solar cells consist of a semitransparent thin film of nanotubes conformally coated on a n-type crystalline silicon substrate to create high-density p-n heterojunctions between nanotubes and n-Si to favor charge separation and extract electrons (through n-Si) and holes (through nanotubes). So, plus strong acid doping, using aligned single wall carbon nanotube film can further improve power conversion efficiency.
Carbon nanotubes as a transparent electrode
Indium tin oxide (ITO) is currently the most popular material used for the transparent electrodes in OPV devices but it has a TON of errors like not being very compatible with polymeric substrates because of its high deposition temperature. Traditional ITO also has unfavorable mechanical properties like being relatively fragile.
Conductive CNT coatings can be an awesome substitute based on a wide range of methods like spraying, spin coating, casting, layer-by-layer, etc. The transfer from a filter membrane to the transparent support using a solvent or in the form of an adhesive film is another method.
CNTs in dye-sensitized solar cells
Due to the simple fabrication process, low production cost, and high efficiency, there is significant interest in dye-sensitized solar cells (DSSCs). It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.
Titanium dioxide nanoparticles have been widely used as a working electrode for DSSCs because they provide high efficiency, more than any other metal.
The transport of electrons across the particle network has been a key problem in achieving higher photoconversion efficiency in nanostructured electrodes. Because electrons encounter many grain boundaries during the transit and experience a random path, the probability of their recombination with oxidized sensitizer is increased.
We can use various CNT based nanostructures to direct the flow of photogenerated electrons and assist in charge injection and extraction. To assist the electron transport to the collecting electrode surface in a DSSC, a popular concept is to utilize CNT networks as support to anchor light harvesting semiconductor particles.
These are just some of the MANY ways in which Carbon Nanotubes are being leveraged for Solar Energy. Things like Carbon Nanotube Hybrids for Renewable Energy or even recently NovaSolix is working on a carbon nanotube-based solar module which has the potential to reach 90% efficiency!!!
There’s so much to look forward too. I really do believe that in the next 5–10 years we will be using Carbon Nanotubes in a lot of our products including for many renewable energy sources like solar.
Alright, this is what I’m getting at:
Hi. I’m Alishba.
I’m so excited to be working in the space of clean energy + sustainability using Nanotechnology, for the next few months. I’d love to connect with you and chat more about the tech + research I’m doing. Feel free to reach out!